The term “granular material” refers to some granular to powdery, easily pourable solid matter, for example grit or fine sand, such as is used for the improvement of the braking effect between the wheels of vehicles and the roadway. For instance, in the case of rail-bound vehicles, granular material or sand from a container is conveyed via a metering and conveying device via a nozzle in front of the rail wheels and into the gap between rail wheel and rail, to increase the frictional resistance between the rail wheel and the rail and reduce the breaking distance.
The EP 2 326 519 B1 describes such a sand metering device for sand spreading systems for vehicles, wherein metering of the sand is effected via an electrically operable stroke magnet. The granular material or the sand can be conveyed downstream of the metering device in various ways, e.g. mechanically or pneumatically.
The DE 297 21 340 U1 shows a sand metering device which is arranged on the sand storage container, comprising a valve which is provided in the outlet opening and is triggered via compressed air. When the valve is open, sand flows from the sand storage container through the outlet opening.
The FR 593 382 A shows a compressed-air actuated sand metering device arranged in a sand storage container, wherein the outlet opening is closed by a valve from the bottom side.
From DE 34 10 409 A1 there is known a device for dispensing sand for rail vehicles, comprising a laterally movable metering piston and a compressed-air supply for supplying the metered sand to the desired location, in particular in front of the wheel of the rail vehicle. The entire weight of the sand rests on the sand inlet, which is why in particular in the case of wet sand blockades may occur. Furthermore, moisture can enter the sand container and lead to clumping together of the sand or at least to a deterioration of the flowability of the sand. The FR 813 969 shows a construction of a metering device, in which the inlet is arranged below the container for the granular material and is closed by a cover. In this case, too, the entire force of the granular material in the container for granular material acts in a disadvantageous way on the inlet or the closing cover, thus complicating the actuation of the cover and not making possible an exact metering.
Further sand metering devices that are arranged inside the sand storage container and are operated by means of compressed air are disclosed in GB 857,246 A or EP 656 292 A1, respectively.
What is a disadvantage in some known metering devices is that different energy forms and energy sources, for example, electrical energy for operating the stroke magnet of the metering device and compressed air for conveying the granular material are required. Since in particular in older rail-bound vehicles or other systems that require metering and conveying of a granular material often only compressed air is available as energy source, some electrically working metering devices are ruled out.